1. Field of the Invention
The present invention relates to an array antenna apparatus for use in spread spectrum communications, wherein the array antenna apparatus includes a plurality of antenna elements aligned on a straight line, and wherein the array antenna apparatus is provided for use in a receiving station which receives a spread-spectrum modulated radio signal having a wavelength of a predetermined carrier frequency transmitted from transmitting stations, using a two-dimensional RAKE receiving method, and which performs spread spectrum communications in code division multiple access.
2. Description of the Prior Art
In conventional spread spectrum communication methods, there has been available such a technique (a so-called RAKE receiving method) such that, by heightening a spreading ratio of spread-spectrum modulation and by broadening a frequency bandwidth (spreading bandwidth) of a spread-spectrum modulated signal, namely, by sufficiently shortening the chip duration of the spread-spectrum modulated signal relative to the change in the delay time (delay broadening or delay range) of multi-path waves, the multi-path wave signals are separated into individual wave signals as delayed pulses on the delay time base by the despread-spectrum technique, using a cross correlation between received signals and spreading codes, and then, the separated delayed pulses are combined. When enough spreading bandwidth cannot be taken so that multi-path waves cannot be separated into individual waves only by differences of the delay time, it is effective to adopt such a two-dimensional RAKE receiving method such that the multi-path wave signals are received by an antenna array in which antenna elements are arrayed or aligned on a straight line at an interval of a half-wavelength distance, and then, the multi-path wave signals are separated into individual waves by using both the differences of delay time, and differences of arrival angle (for example, See Takashi Inoue et al., "Channel Capacity Improvement in the Uplink of DS/CDMA Systems by Means of 2-Dimensional RAKE Reception Scheme", Technical Report of the Institute of Electronics, Information and Communication Engineers in Japan, A.P97-103, RCS97-118, October 1997).
FIG. 2 is a block diagram showing an implementation of a spread spectrum communication system of a prior art example, and FIG. 3 is a perspective view showing the concept of a two-dimensional RAKE receiving method of a prior art example.
Referring to FIG. 2, transmitting stations 100-1 to 100-K are equipped with data modulation sections 1-1 to 1-K, spreading modulation sections 2-1 to 2-K, RF transmitting sections (radio frequency transmitting sections) 3-1 to 3-K, and transmitting antennas 4-1 to 4-K, respectively. Spread spectrum signals S1-1 to S1-K transmitted from the plurality of K transmitting stations 100-1 to 100-K, respectively, arrive at a receiving station 200 via a multi-path transmission line 300.
In the receiving station 200, the signals are received by an array antenna 500 comprising a plurality of M antenna elements 5-1 to 5-M arrayed or aligned on a straight line at an antenna element interval D of, for example, a half-wavelength (.lambda./2). Individual received signals S2-1 to S2-K are converted into intermediate frequency signals or baseband frequency signals by RF receiving sections (radio frequency receiving sections) 6-1 to 6-M, respectively, and then, the converted signals are converted into M types or kinds of beam space signals S3-1 to S3-M by a multi-beam forming circuit 7. In this case, the multi-beam forming circuit 7 as shown in FIG. 4 is a well circuit configuration and comprises an embodiment where eight beam space signals S31-1-S3-8 are generated based on eight input signals S1-1-S1-8 as an example. The multi-beam forming circuit 7 comprises:
(a) 180.degree. phase shifters PS11 to PS14, PS21 to PS24, PS41 and PS44, 90.degree. phase shifters PS31 to PS32, 1350 phase shifters PS33 and PS34, and 215.degree. phase shifters PS35 and PS36; and PA1 (b) in-phase combiners (or adders) AD11 to AD18, AD21 to AD28 and AD31 to AD38.
Next, the individual beam space signals S3-1 to S3-M outputted from the multi-beam forming circuit 7 are distributed or divided into a plurality of K signals, and then, the divided K signals are inputted to K two-dimensional RAKE receiving sections 8-1 to 8-K, respectively. For example, the two-dimensional RAKE receiving section 8-1 of the first user channel, as shown in FIGS. 2 and 5, comprises a plurality of M despreading circuits 811-1 to 811-M, a plurality of M RAKE receiving circuits 812-1 to 812-M, a combining circuit 813, and a data demodulation section 814. In the two-dimensional RAKE receiving section 8-1 of the first user channel, the distributed m-th (m=1, 2, . . . , M) beam space signal S3-m of the first user channel is despread by the despreading circuit 811-m, and a RAKE combined signal S41-m of the m-th beam in the first user channel is generated by the RAKE receiving circuit 812-m. The RAKE combined signals S41-1 to S41-M of the first to M-th beams in the first user channel are maximum-ratio combined so as to generate a two-dimensional RAKE combined signal S5-1 of the first user channel. After that, the generated two-dimensional RAKE combined signal S5-1 of the first user channel is demodulated so as to generate a demodulated signal S6-1 of the first user channel by the data demodulation section 814. The other two-dimensional RAKE receiving sections 8-2 to 8-K of the second to K-th user channels also operate in a similar manner, so as to generate demodulated signals S6-2 to S6-K of the second to K-th user channels, respectively.
That is, since the two-dimensional RAKE receiving sections 8-1 to 8-K obtain their outputs by maximum-ratio combining the input signals in a two-dimensional domain of time and space, the multi-path waves can be separated into individual waves with both differences of delay time and differences of arrival angle, this results in advantageous effects such that higher-quality data transmission can be realized by efficiently separating multi-path waves.
FIG. 6 is a plan view showing a multi-path transmission line of the spread spectrum communication system of FIGS. 1 and 2. FIG. 6 shows only one transmitting station 100 and one receiving station 200, wherein spread-spectrum radio signals transmitted from the transmitting stations 100 are received by the receiving station 200 via, for example, seven paths P0 to P6. In this case, the receiving station 200, as shown in FIG. 7, receives spread spectrum radio signals with a delay range.
In a case of less broadening of the arrival angle of multi-path waves that arrive at the receiving station 200, in order to separate the individual multi-path waves based on differences of arrival angle, it is necessary to utilize an array antenna having a very large number of antenna elements. In other words, the hardware scale becomes relatively large.